What is meant by the enthalpy change of a reaction?

The enthalpy change of a reaction is the heat energy taken in or given out when the reaction occurs at constant pressure, for the molar quantities shown in the balanced equation.

What does a positive value of ΔH tell you about a reaction?

A positive ΔH means the reaction is endothermic: energy is taken in from the surroundings, the surroundings get colder, and the products have a higher enthalpy than the reactants.

Why does the combustion of a fuel have a negative enthalpy change, in terms of the bonds involved?

Breaking the bonds in the fuel and in oxygen takes in energy, while forming the bonds in carbon dioxide and water gives energy out. In combustion the energy released on forming the new bonds is greater than the energy needed to break the old ones, so there is a net release and ΔH is negative.

State the standard conditions used when quoting a standard enthalpy change.

A pressure of 100 kPa, a stated temperature which is normally 298 K, every substance in its standard physical state at that pressure and temperature, and any solution at a concentration of 1 mol dm⁻³.

Define the standard enthalpy change of combustion of a substance.

It is the enthalpy change when one mole of the substance is burned completely in oxygen, with all reactants and products in their standard states under standard conditions.

WJEC AS-Level Chemistry 1410QS

Formulae and Equations

Topic overview Charges on common ions Building formulae for ionic and molecular compounds Assigning oxidation numbers Using oxidation numbers to follow redox changes Balancing equations and using state symbols Ionic and half equations

Basic Ideas About Atoms

Topic overview Radioactive decay and nuclear equations Alpha, beta and gamma radiation Half-life, dating and the uses of radioisotopes: half-life of radioactive decay Half-life, dating and the uses of radioisotopes: adverse consequences for living cells Ionisation energies and electronic structure Orbitals and the filling of sub-shells Atomic spectra and the energy of light: atomic emission spectrum of the hydrogen atom Atomic spectra and the energy of light: relationship between energy and frequency Atomic spectra and the energy of light: significance of the frequency

Chemical Calculations

Topic overview Relative masses and the mass spectrometer: relative atomic Relative masses and the mass spectrometer: how a mass spectrometer works Relative masses and the mass spectrometer: molecular ion The mole, the Avogadro constant and molar mass: relative mass terms and simple mass spectra The mole, the Avogadro constant and molar mass: moles and percentage composition by mass Empirical and molecular formulae Concentration, solubility and titration calculations: concentration in mol dm-3 and in g dm-3 Concentration, solubility and titration calculations: stoichiometry and the limiting reagent Concentration, solubility and titration calculations: titration calculations and estimating a titre Gas volumes and the ideal gas equation Yield, atom economy and experimental uncertainty: concepts of atom economy and percentage yield Yield, atom economy and experimental uncertainty: how to estimate the percentage error

Bonding

Topic overview Ionic bonds and covalent bonds: ionic bonding in terms of ion formation Ionic bonds and covalent bonds: covalent bonding and forces Electronegativity, polarity and in-between bonds: concepts of electronegativity and bond polarity Electronegativity, polarity and in-between bonds: forces Forces between molecules Hydrogen bonding Shapes of molecules and ions: VSEPR principle and its use Shapes of molecules and ions: bond angles associated with linear

Solid Structures

Topic overview Ionic crystals: sodium chloride and caesium chloride Giant covalent structures: diamond and graphite Simple molecular solids: iodine and ice Metals and the electron sea Using structure to explain physical properties

The Periodic Table

Topic overview How the Periodic Table is organised Oxidation and reduction as electron transfer Trends across periods and down groups Group 2: reactions, trends and the reasons behind them — reactions with oxygen, water and dilute acid Group 2: reactions, trends and the reasons behind them — trends down the group Group 2: reactions, trends and the reasons behind them — thermal stability of the carbonates Group 2: reactions, trends and the reasons behind them — oxides and hydroxides Identifying ions, making salts and gravimetric analysis: identifying cations with sodium hydroxide Identifying ions, making salts and gravimetric analysis: preparing a soluble salt from an acid Group 7: the halogens — physical properties and the trend in volatility Group 7: the halogens — reactions of the halogens with metals Group 7: the halogens — displacement reactions and the trend Group 7: the halogens — halide ions as reducing agents, uses of chlorine and the chlorate equilibrium

Simple Equilibria and Acid-Base Reactions

Topic overview Reversible reactions and dynamic equilibrium Le Chatelier's principle and shifting the position of equilibrium The equilibrium constant Kc Acids, bases, salts and neutralisation pH, strong acids and weak acids Acid-base titrations and their calculations

Thermochemistry

Topic overview Enthalpy changes and their definitions Calculating enthalpy changes from experimental measurements Determining enthalpy changes in the laboratory Hess's law and energy cycles Average bond enthalpies

Rates of Reaction

Topic overview Collision theory and the conditions that change rate Measuring rate and relating it to concentration Energy profiles and activation energy Temperature and the Boltzmann distribution Catalysts and how they work Following reactions in the laboratory

The Wider Impact of Chemistry

Topic overview Social and economic consequences of chemical manufacture and energy supply Environmental consequences of chemical processes and energy production Green chemistry and sustainable practice

Organic Compounds

Topic overview Drawing and reading organic structures Naming organic compounds Chain length, functional groups and physical properties Structural isomerism Electrophiles, nucleophiles, radicals and bond fission

Hydrocarbons

Topic overview Alkane bonding, combustion and fossil fuels: combustion reaction of alkanes and benefits Alkane bonding, combustion and fossil fuels: combustion calculations and acid rain Radical substitution of alkanes The C=C bond and why alkenes are reactive E–Z isomerism in alkenes Electrophilic addition, Markovnikov's rule and testing for alkenes: mechanism of electrophilic addition Electrophilic addition, Markovnikov's rule and testing for alkenes: Markovnikov's rule and the bromine water test Electrophilic addition, Markovnikov's rule and testing for alkenes: testing for a C=C bond Hydrogenation and addition polymers: conditions required Hydrogenation and addition polymers: nature of addition polymerisation

Halogenoalkanes

Topic overview Nucleophilic substitution by hydroxide ions Elimination to form alkenes Reactivity of the carbon–halogen bond and the silver nitrate test Uses of halogenoalkanes, CFCs and the ozone layer: uses of halogenoalkanes and CFCs Uses of halogenoalkanes, CFCs and the ozone layer: radical mechanism Uses of halogenoalkanes, CFCs and the ozone layer: replacements for CFCs Heating under reflux in the laboratory

Alcohols and Carboxylic Acids

Topic overview Two routes to ethanol: hydration and fermentation — hydration of ethene Two routes to ethanol: hydration and fermentation — fermentation Two routes to ethanol: hydration and fermentation — comparing the two routes on rate Classifying alcohols and dehydrating them Oxidising alcohols to aldehydes, ketones and acids Chemical tests for alcohols and carboxylic acids Carboxylic acids, bases and salts Making an ester and separating it by distillation: esterification Making an ester and separating it by distillation: carrying out the preparation Making an ester and separating it by distillation: separating and purifying Making an ester and separating it by distillation: yield

Instrumental Analysis

Topic overview Reading a mass spectrum Infrared spectra and functional groups NMR spectra and atomic environments Putting the three techniques together

Practical Skills and Techniques

Topic overview Measuring mass, volume, time and temperature Standard solutions and titration technique Indicators and acid–base titration curves Heating, refluxing, distilling and purifying: Heating Heating, refluxing, distilling and purifying: purifying Melting temperature, chromatography and pH measurement Electrochemical cells, rate methods and working safely: Electrochemical cells Electrochemical cells, rate methods and working safely: rate methods and working safely

Planning, Analysis and Evaluation

Topic overview Planning an investigation and controlling variables Recording observations and presenting data Graphs, gradients and processing data Errors, uncertainty, accuracy and precision Evaluating a method and suggesting improvements Research and referencing

Core Experiments and Their Chemistry

Topic overview Gravimetric and acid–base analysis Enthalpy and rate experiments: Enthalpy Enthalpy and rate experiments: rate experiments Redox titrations and electrochemical cells Equilibrium constants and following a titration with a pH probe Organic synthesis, separation and identification: Organic synthesis Organic synthesis, separation and identification: identification tests and chromatography

Mathematical Skills in Chemistry

Topic overview Units, standard form and proportion in chemical calculation: volume, equilibrium, rate and energy units Units, standard form and proportion in chemical calculation: standard form Units, standard form and proportion in chemical calculation: proportion Units, standard form and proportion in chemical calculation: checking answers, buffers, rate constants and units Significant figures, means and uncertainty in experimental data: significant figures in multiplication Significant figures, means and uncertainty in experimental data: relative atomic mass as a weighted mean Significant figures, means and uncertainty in experimental data: absolute and percentage uncertainty Symbols, rearranging equations and logarithms: chemical symbols and what each one asserts Symbols, rearranging equations and logarithms: rearranging an equation Symbols, rearranging equations and logarithms: substituting into equations for Kc, enthalpy and pH Symbols, rearranging equations and logarithms: logarithms, the Arrhenius equation and rearranging formulae Graphs: plotting, gradients, intercepts and tangents — reading a spectrum Graphs: plotting, gradients, intercepts and tangents — reaction order from data and plotting good graphs Graphs: plotting, gradients, intercepts and tangents — Arrhenius plots, zero-order graphs and tangents Graphs: plotting, gradients, intercepts and tangents — initial rates from tangents and interpreting graphs Shape, symmetry and three-dimensional structure: Shape Shape, symmetry and three-dimensional structure: symmetry

Thermochemistry

5 subtopics
15 things to know
AS Unit 2

What Thermochemistry covers

Thermochemistry is examined on AS Unit 2 of WJEC Chemistry, Energy, Rate and Chemistry of Carbon Compounds. 80 recall questions across 5 subtopics.

Where it is assessed

AS Unit 2

Energy, Rate and Chemistry of Carbon Compounds.

Check the WJEC specification ↗

The 5 subtopics

Each one has its own page. WJEC AS Chemistry students sit this topic in the first year.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why is the standard enthalpy change of formation of oxygen gas zero?

Key term

Why can enthalpy changes of formation not be measured directly for most organic compounds?

Numbers

State the equation used to calculate the heat energy transferred to a known mass of water, and define each term.

Evaluation

Why is a temperature change in degrees Celsius the same number as the temperature change in kelvin?

Key term

Describe how you would determine the enthalpy change of combustion of a liquid fuel using simple laboratory apparatus.

Key term

Why is a polystyrene cup a better calorimeter than a glass beaker for a neutralisation experiment?

Key term

Why is a lidded polystyrene cup used as a calorimeter, and how is heat lost from a spirit burner?

Key term

Which fundamental principle is Hess's law a consequence of?

Key term

When constructing an energy cycle using enthalpy changes of formation, in which direction do the arrows point?

Numbers

Calculate the enthalpy change for 2Al(s) + Fe₂O₃(s) → Al₂O₃(s) + 2Fe(s), given ΔfHꝊ(Fe₂O₃) = ‒824 kJ mol⁻¹ and ΔfHꝊ(Al₂O₃) = ‒1676 kJ mol⁻¹.

Numbers

Calculate the enthalpy change for H₂(g) + Cl₂(g) → 2HCl(g) using these average bond enthalpies in kJ mol⁻¹: H‒H 436, Cl‒Cl 243, H‒Cl 432.

Numbers

Calculate the enthalpy change for H₂(g) + Br₂(g) → 2HBr(g) using these average bond enthalpies in kJ mol⁻¹: H‒H 436, Br‒Br 193, H‒Br 366.

Numbers

Calculate the enthalpy change for N₂(g) + 3H₂(g) → 2NH₃(g) using these average bond enthalpies in kJ mol⁻¹: N≡N 944, H‒H 436, N‒H 391.

Key term

What does a negative value of ΔH tell you about a reaction?

Key term

Why is the enthalpy change of a reaction negative when the temperature of the reaction mixture rises?

Take Thermochemistry further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

As a source of error, incomplete combustion vs As a source of error, heat loss
As a source of error, incomplete combustion: less energy was actually released by the fuel. As a source of error, heat loss: it means the energy released never reached the water.

Questions from this topic

Each one with the model answer underneath.

1
What is meant by the enthalpy change of a reaction?
Model answer
The enthalpy change of a reaction is the heat energy taken in or given out when the reaction occurs at constant pressure, for the molar quantities shown in the balanced equation.
2
What does a negative value of ΔH tell you about a reaction?
Model answer
A negative ΔH means the reaction is exothermic: energy is transferred from the chemicals to the surroundings, the surroundings get warmer, and the products have a lower enthalpy than the reactants.
3
What does a positive value of ΔH tell you about a reaction?
Model answer
A positive ΔH means the reaction is endothermic: energy is taken in from the surroundings, the surroundings get colder, and the products have a higher enthalpy than the reactants.
4
Why does the combustion of a fuel have a negative enthalpy change, in terms of the bonds involved?
Model answer
Breaking the bonds in the fuel and in oxygen takes in energy, while forming the bonds in carbon dioxide and water gives energy out. In combustion the energy released on forming the new bonds is greater than the energy needed to break the old ones, so there is a net release and ΔH is negative.
5
State the standard conditions used when quoting a standard enthalpy change.
Model answer
A pressure of 100 kPa, a stated temperature which is normally 298 K, every substance in its standard physical state at that pressure and temperature, and any solution at a concentration of 1 mol dm⁻³.
6
Define the standard enthalpy change of combustion of a substance.
Model answer
It is the enthalpy change when one mole of the substance is burned completely in oxygen, with all reactants and products in their standard states under standard conditions.

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Formulae and Equations

Basic Ideas About Atoms

Chemical Calculations

Bonding

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An element is zero.

uncombinedionisedoxidised

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AVO₂⁺ + 2H⁺ + e⁻ → VO²⁺ + H₂O.

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COne electron is gained.

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Thermochemistry revision questions

Is Thermochemistry examined at AS in WJEC Chemistry?: Yes. It sits in AS Unit 2, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.
What does Thermochemistry cover in WJEC AS-Level Chemistry?: 5 subtopics: enthalpy changes and their definitions, calculating enthalpy changes from experimental measurements, determining enthalpy changes in the laboratory, hess's law and energy cycles and average bond enthalpies. Each one has its own page with questions and model answers.
How is Thermochemistry organised?: By the WJEC 1410QS specification, subtopic by subtopic, so it matches the order you are taught it in.

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